Known navigation devices perform routing functions. For example, Japanese Kokai Patent Application No. 2001-349735 teaches a navigation device wherein when a traffic restriction or traffic jam or the like arises on the route to a destination, the device guides a vehicle to a detour route at a predetermined distance or time from the site where the traffic restriction or traffic jam has arisen.
In the conventional navigation device, however, problems can arise because the driver may not be able to enter the optimal detour route because if the predetermined distance or time is short, guidance to a detour route can be provided after passing the branching point to the detour route.